Photo-electrochemical cells consist of a semiconductor electrode immersed into a suitable redox electrolyte together with a reversible metal electrode. Upon illumination the holes and electrons generated by the incident photons are separated by the potential difference over the depletion layer and react with the redox couple in the electrolyte. A schematic of the working of such cells is shown in FIG. 1. For example, in case of an n-type semiconductor, the photo-holes move to the surface where they react with a reducing agent (e.g. Fe(CN)64−+h+→Fe(CN)63−). The electrons move through the external electrical circuit towards the reversible counter electrode (e.g. Pt), where they reduce the oxidizing agent (e.g. Fe(CN)64−+e−→Fe(CN)64−). In this way, the light energy is converted into electrical energy and since the redox system is reversible, the cell can in principle go on forever.
Photo-electrochemical cells form components of several devices, such as sensor devices, solar-energy devices, and detection devices and of chemical installations for synthesis and electrocatalysis such as electrolysis. The structures of preferred embodiments be used as an electrochemical cell component for all above applications, with adjustments and additions as necessary. An example of photoelectrochemical cell for solar-energy conversion is the n-type Si semiconductor functionalized with CNT carpet as described in this patent and used as the photo-anode in a cell with an inert platinum electrode, more particularly a transparent ITO electrode functionalized with platinum particles, and an electrolyte with a stable redox couple such as Fe3+/Fe2+, preferably in a complexed form such as the iron(III)hexacyanoferrate (Fe(CN)63−)/iron(II)hexacyanoferrate (Fe(CN)64−) redox couple. An example of a device for photo-electrochemical detection is a device for detection of separated species coming from an HPLC column [e.g. photo-electrochemical detection at a TiO2 wire by G. N. Brown, J. W. Birks, C. A. Coval, in Analytical Chemistry, Volume 64, page 427, published in 1992]. An example of a sensor device could be for the photo-electrochemical detection of enzymes or proteins in biological samples. Here, both the photocurrent and photovoltage could be used as detection signals. For the sensor application, it may be necessary to further functionalize the CNT which are on the semiconductor surface so that the photoelectrochemical cell becomes selective to certain molecules.
Semiconductor materials and especially low band-gap semiconductor materials such as Si, Ge and III-V materials are unstable in wet environment, aqueous and non-aqueous, especially under current flow. Silicon easily forms an oxide in the presence of water. When using Si as an anode for electro-oxidation reactions, silicon itself will oxidize and form an insulating or passivating layer, shutting down the electrochemical reactions as shown schematically in FIG. 2. Germanium and III-V semiconductors with Ga, In and Al as III-elements and As, Sb and P as V-elements (e.g. GaAs, InP, InSb, GaSb, GaP, InAs, AlxGayAs, InxGayAs and other ternary and quaternary combinations) are unstable in electrolyte solutions and will decompose under current flow. Under anodic current, the semiconductor itself will be electro-etched and the semiconductor material will dissolve. Also under cathodic current flow electrode decomposition can occur (e.g. “Cathodic decomposition and anodic dissolution and changes in surface morphology of n-type InP in HCl” by M. Seo, M. Aihara, A. W. Hassel; in JOURNAL OF THE ELECTROCHEMICALSOCIETY Volume: 148 Issue: 10 Pages: B400-B404 Published: October 2001). The severity of the electro-dissolution is dependent on solution composition and the presence of stabilizing agents [e.g. “The effect of high LiCl concentrations upon the competition between anodic decomposition and stabilization of n-GaAs/Fe2+ electrode”, by K. Strubbe and W. P. Gomes, in JOURNAL OF ELECTROANALYTICAL CHEMISTRY Volume: 349 Issue: 1-2 Pages: 429-441 Published: Apr. 30, 1993]. The stabilizing agents are for example suitable reducing agents which will compete with the electro-etching reaction and thus slow down the electrode decomposition kinetics. These stabilizing agents can however not prevent decomposition completely and the electrode will always be slowly consumed.
The use of silicon in photoelectrochemical cells (e.g. electrochemical sensors or solar cells) is of interest because of the suitable band gap of 1.12 eV and because of its widespread availability and supporting technologies. The surface of silicon is very chemically active though and will easily react with water or oxygen, forming an insulating oxide. This oxide on the surface can impede the characteristics of the silicon. Performances of electrochemical cells based on silicon electrodes will hence gradually deteriorate over time due to oxide growth.
FIG. 3 shows an example of the electrochemical response (current-voltage curves) for a Si photoanode in an electrolyte solution with Fe(CN)63−/Fe(CN)64− as a suitable redox couple: In the first voltage scan towards positive potentials an anodic photo-current plateau is measured for the oxidation of Fe(CN)64− (valence of +II) to Fe(CN)63− (valence of +III). Already in the reverse scan (back towards more negative potentials), the current-voltage curve (i-V curve) has shifted towards more positive potentials due to the oxidation of the silicon surface, showing that not all the photo current was used for the oxidation of the reducing agent only. The current-voltage behaviour deteriorates even more upon subsequent scans as the i-V curves shift towards more positive voltage values upon each scan. Also the cathodic currents deteriorate as the oxide also hinders the reduction reactions.
Even in organic media, small amounts of water present will eventually passivate the surface. This is illustrated in FIG. 4 where the current-voltage characteristics of an n-type Si photo-anode are shown in acetonitrile medium. The Si surface slowly deteriorates in the acetonitrile solution upon several scans for photo-anodization of the I− ions to form I2. The oxidation of Si is sped up drastically by the addition of small amounts of water to the organic acetonitrile solution.
Therefore there is a need to stabilize Si electrode surfaces such that oxidation (anodization) is avoided (eliminated), especially when working in aqueous media.
A typical way of forming a clean silicon surface is to use wet chemical etching with HF(aq.), resulting in a hydrogen-terminated Si surface. This surface is electrically suitable when initially formed. However, this surface readily oxidizes in air or in water-containing ambients. That overlayer can impede the semiconductor characteristics. For example, this can impede photocurrent flow through an electrochemical cell.
Previous art has suggested coating the surface of Si electrodes with islands or films of metal. This process creates buried Si/metal junctions on protected regions of the electrode which is to be avoided.
U.S. Pat. No. 6,759,349 stabilizes silicon surfaces by depositing a self assembled monolayer (SAM) onto the silicon surface, whereby the SAM layer contains derivatives with covalently-attached alkyl chains used to protect the surface. A preferred mode uses a halogenation/alkylation procedure in which the surface is treated with a halogen, e.g., chlorine, and then an alkyl-containing material. The problem however with these SAM layers is the fact that these layers have a negative impact on the conductivity. Furthermore these SAM layers tend to deteriorate in time or released from the Si-surface.
Another solution proposed in the prior art methods and one of the straightforward approaches is to avoid oxide growth by using a non-aqueous electrolyte in combination with a redox couple, instead of an aqueous electrolyte with redox couple. However, the organic solvent will not be absolutely free of water even after treatment with drying agents or conditions which means that the problem is not solved.